Cigarette filter from polypropylene fibers

ABSTRACT

A roving of entangled and self-bonded fine fibers of polypropylene may be formed by a melt blown roving technique which comprises extruding the polypropylene through a die having the die openings in a circle into a gas stream to attenuate the extruded polypropylene into fibers and collecting the fibers as a tow. The roving or tow of polypropylene fibers is an aggregation of fiber loops which is essentially cylindrical in shape, made from essentially continuous fibers which are entangled and bonded to each other. The roving or tow may be cut as filters for cigarettes.

United States Patent Robert R. Buntin;

John W. Harding; James P. Keller; Vollie L. Murdock, all 01 Baytown,Tex. 752,655

Aug. 14, 1968 July 27, 1971 Esso Research and Engineering Company I 72]Inventors [21] Appl. No. [22] Filed [45] Patented [73] Assignee [54]CIGARETTE FILTER FROM POLYPROPYLENE rmr-zns IIChImsJDru/IngFigs.

s21 u.s.c1 131/269,

1s6/161,1s6/1so,161/150,161/179,131/261 B 5 1 .lIIl-CI A24f7/04,

' BOld 27/00 501 FleldoiSareh 131/269,

[56] References Cited UNITED STATES PATENTS 2,719,350 10/1955Slayteretal. l56/167X FIBER FROM DIE POINT OF CONTACT STABILIZER ROD2,966,157 12/1960 Touey et al. 131/269 3,148,101 9/1964 Allman etal.131/269 UX 3,219,043 11/1965 Erlich et a1. 131/269 X 3,444,863 5/1969Soehngen et al 131/269 X Primary Examiner- Samuel Koren AssistantExaminer-:0. M. Yahwak AnorneysThomas B. Mc Culloch, Melvin F. Fincke,John B. Davidson, John S. Schneider, Sylvester W. Brock, Jr. and Kurt S.Myers ABSTRACT: A roving of entangled and self-bonded fine fibers ofpolypropylene may be formed by a melt blown roving technique whichcomprises extruding the polypropylene through a die havingthe dieopenings in a circle into a gas stream to attenuate the extrudedpolypropylene into fibers and collecting the fibers as a tow. The rovingor tow of polypropylene fibers is an aggregation of fiber loops which isessentially cylindrical in shape, made from essentially continuousfilfers which are entangled and bonded to each other. The roving or towmay be cut as filters for cigarettes.

FORWARD TAKE-OFF METHOD POINT OF CONTACT PATENTED JUL27 I97! SHEET 2 Or00m mwkmzhm N OI mm m2 wo @zrSmjou m POI EmQDmPXw INVENTORS.

ROBERT R. BUNTIN,

JAMES P. KELLER, JOHN W. HARDING,

VOLLIE L. MURDOCK,

IiY

M. TTIORNEY.

CIGARETTE FILTER FROM POLYPROPYLENE FIBERS BACKGROUND OF THE INVENTIONThe present invention is directed to a nonwoven roving of polypropylenefibers which may be useful as a filtering material especially as filtersfor cigarettes. The cigarette filter comprises a roving of entangledfine fibers of polypropylene having an average bulk density between 0.05and 0.20 g./ce. A roving of the polypropylene fibers may be producedwhich need not be wrapped with paper to provide the proper circumferenceas a cigarette filter but may be cut and only wrapped with theoutercigarette paper for forming a filter cigarette.

In considering some of the processes of the prior art which disclose theformation of a cylindrical tow of fibers, the following U.S. patentshave been considered U.S.' Nos. 2,886,877; 3,023,075 3,148,101and3,232,805.

SUMMARY OF THE INVENTION The present invention is directed to a rovingof fine fibers which are entangled and self-bonded and made up of fiberloops which form an essentially cylindrical tow. The fibers in theroving are essentially continuous fibers looped back and forth from apoint on a short radius or near the center of the cylindrical tow to apoint on a longer radius or near the outer surface of the cylindricaltow in a fairly uniform manner. The fibers in the roving have a sizebetween approximately 2 and 40 microns. The cylindrical roving hasfibers which are preferably self-bonded and have a smooth skin of bondedfibers on the outer surface of the roving.

The tow or roving may be produced by extruding the resin through a diehaving the die openings arranged in a circle using a melt blown rovingtechnique. Gas streams, preferably air, are supplied by gas plenums inthe form of slots concentrically inside and outside of the circle of dieopenings to attenuate the extruded resin into fine fibers. The circle ofdie openings are arranged in the die so as to extrude and direct theresin as a cone of fibers in space extending from the die openings, thecone of fibers being largest at the die and narrowing in an imaginaryfocal point in front of the die. The shape of the cone of fibers inspace may be modified by controlling the relative velocities of the gasstreams from the plenums inside and outside the circle of die openingsto change the distance of the imaginary focal point ofthe cone of fibersin front ofthe die.

The conical profile of the fibers coming from the die openings may befurther controlled by means of an auxiliary gas stream supplied througha gas plenum in the form of a jet in the center of the circle of dieopenings. The auxiliary gas stream assists in cooling the fibers priorto collection and also modifies the laydown of the fibers. Thus, theauxiliary gas stream may be used to change the conical shape of thefibers as they are attenuated from the die so that the profile of thefibers in space become more cylindrical at the laydown zone. By thuscontrolling the auxiliary gas stream, the time-temperature history ofthe extruded resin may be changed as well as the laydown pattern oftheindividual fibers.

The fibers may be collected as a tow in a number of distinct ways. Thefibers may be collected either by moving the tow of fibers away from thedie or through the center of the circle of die openings. The directionof removal of the tow of fibers formed are distinguished by definingforward takeoff as collecting and moving the tow in a direction awayfrom the die and reverse takeoff as collecting and moving the tow offibers back through the center of the circle of die openings. Thedirection of collecting and moving the tow results in the production ofdistinctly different products. For example, in the reverse takeoff asmooth skin of bonded fibers may be produced on the outside of the tow,while on the other hand, forward takeoff produces a softer outer layerof individual fibers and aggregates of fibers on the tow.

A fixed or rotating collecting means may be used in the laydown zone tocollect the fine fibers in a desired form. The

preferred collecting means is a stabilizer rod. The stabilizer rod ispreferably rotated to produce a uniform tow. The rotation of thestabilizer rod produces a permanent twist in the product and eliminatesthe opening or soft center of the tow due to being removed from thestabilizer rod. However, if a hollow tow is desired, a large stationaryor rotating rod may be used.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of theoverall melt blown roving process with reverse takeoff;

FIG. 2 is a schematic view of the melt blown roving process with forwardtakeoff;

FIG. 3 is a pictorial representation of fiber laydown with reversetakeoff;

FIG. 4 is a pictorial representation of fiber laydown with forwardtakeoff; and

FIG. 5 is a schematic view of the melt blown roving process with aplurality of dies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 which setsforth one embodiment of the overall melt blown roving process, athermoplastic such as polypropylene is introduced into the feed hopper Iof an extruder 2. The thermoplastic is heated in the extruder 2 and isplaced under shear by a screw (not shown) in the extruder 2 driven bydrive 3. The thermoplastic is extruded from a die 4 which has aplurality of die openings 5 arranged in a circle into a gas stream. Thegas stream is supplied by gas plenums 6 and 7 arranged concentricallyinside and outside, respectively, to the circle of die openings 5. i

The thermoplastic is extruded from the die 4 as continuous fibers whichare attenuated by the gas stream from plenums 6 and 7. Gas is suppliedto the plenums 6 and 7 by lines 8 and 9, respectively, each of which maybe independently controlled as to pressure and temperature (not shown).By the control of the gas stream, the extruded fibers are attenuatedfrom the die openings 5 in the shape of a cone, with the largest portionof the cone being at the die openings 5. The fibers are collected in alaydown zone 10 which is in proximity to the imaginary apex of the coneof attenuated fibers. The distance from the die openings 5 to thelaydown zone 10 is controlled to a large extent by the relative gasvelocities from plenums 6 and 7.

The attenuated fibers are collected on a collecting device 11 which mayhave various shapes and sizes. The collecting device ll is preferably astabilizer rod 12 which is held stationary or rotated. The stabilizerrod 12 may be placed through the center of a funnel 13 or other meanswhich act as a bearing to stabilize the rod 12 when rotated by a drivemeans 14. The funnel 13 also protects the drive means 14 from stray finefibers. As the fibers collect on the stabilizer rod 12, they entangleand become self-bonded.

To modify the configuration of the fibers as they are col- "lected inthe laydown zone 10 on the stabilizer rod 12, an auxiliary gas streammay be supplied from a gas plenum 15 formed by a double-wall pipe 16.The double-wall pipe 16 is positioned along the axis defined by thecenter axis ofthe die 4 and the center of the circle of die openings 5.The gas stream from gas plenum 15 is directed at the collecting device11 or stabilizer rod I2 and the air stream modifies the laydown of thefibers in laydown zone 10 so that they take a more cylindrical shape.Gas, preferably air, is supplied to the double-wall pipe 16 by line 17which may be controlled as to pressure and temperature. The auxiliarygas stream may be used to modify the time-temperature history of thefibers as well as the configuration and entanglement of the fibers. Thegas supplied through gas plenum 15 may be cooler or hotter than the airsupplied through plenums 6 and 7.

To begin the melt blown roving operation, a starter rod 18 is extendedinto the laydown zone 10 along the stabilizer rod 12 to draw the fibersas a continuous tow from the stabilizer rod 12 either in a forwardtakeoff or in a reverse takeoff, the

reverse takeoff illustrated In FIG I As shown In FIG 1. the starter rod18 is drawn through the center of the double-wall pipe 16 where the towis attached to a windup reel (not shown).

The melt blown roving operation of the present invention allows numerousproducts to be produced by varying the conditions in the laydown zone10. The fibers are extruded and attenuated from the die openings 5initially in the form ofa cone A. As the fibers converge, the individualfibers come in contact with one another and with the roving previouslyformed. Being hot the fibers can stick or self-bond and begin toentangle. The fibers, however, continue to. attenuate and stick togetherand to the roving further from the die openings 5 in the laydown zoneuntil a mass of entangled and self-bonded fibers is fonned. The laydownzone 10 extends from the point where initial contact of fibers occur tothe furthest point away from the die openings 5 where the fibers collectin the fiber mass as a tow. The laydown zone 10 may be several inches toa foot or so from the die openings 5. The laydown zone 10 may bemodified by independently varying the angle at which the extrudedthermoplastic comes from the die in relation to the axis of the die, theuse of the auxiliary gas stream from plenum and the direction oftakeoff. The use of the auxiliary gas stream from plenum 15 in mostinstances produces a more uniform laydown of the fibers and accordinglya more uniform tow of fibers is produced. When the tow 19 is removedfrom the laydown zone 10 by a reverse takeoff as illustrated in FIG. 1,the tow may have a bonded and smooth outer surface, whereas when the towis removed from the laydown zone 10 in a forward takeoff, the outersurface appears more fluffy. A tow may be produced which is less denseand has less selfbonding by having the laydown zone 10 further away fromthe die openings 5.

Referring to FIG. 2, the melt blown roving technique of the presentinvention is illustrated with the forward takeoff. The thermoplasticresin is introduced into the feed hopper 20 of an extruder 21. Thethermoplastic is heated, placed under shear by a screw in the extruder21 driven by drive 22 and extruded from a die into a gas stream. The diehas a plurality of die openings 24 arranged in a circle. The gas streamis supplied by gas plenums 25 and 26 arranged concentrically inside andoutside, respectively, each of which may be independently controlled asto pressure and temperature. The extruded fibers are attenuated from thedie openings 24 by the gas stream and collected in a laydown zone 29.

The collecting device 30 preferred in the forward takeoff embodiment isa stabilizer rod 31 which is rotated by drive 32. The stabilizer 31 isinserted through the center of a doublethere IS entanglement and somebonding between fibers so that the continuous looping from centerline ofthe roving to the outside surface is not always complete. When theroving made by the melt blown roving technique is pulled apart, however, the roving breaks substantially along the lines set forth as aloop to give a cone and conical socket break.

In a specific application, the melt blown polypropylene roving makes agood filter for cigarettes. One of the requirements for a commercialcigarette filter material is that it be supplied as a continuous towthat can be readily converted to the cigarette filter or as a firm,rodlike product. Thus, for a cigarette filter the circumference of thetow may be within a range of 24 to 28 mm. The bulk density of thepolypropylene tow of the present invention may vary, however, issuitable as a cigarette filter between the range of 0.05 and 0.20 g./cc.and preferably between 0.09 and 0.16 g./cc.

In FIG. 5 the melt blown roving technique is illustrated with aplurality of dies. By using two or more dies, tows may be producedhaving distinct and unusual properties. The thermoplastic resin such aspolypropylene may be introduced into hoppers 40, and of extruders 41,51,and 61, respectively. The resin is extruded out of the dies 42, 52, and62. As illustrated, the first die 42 utilizes forward takeoff with astabilizer rod 43 rotated by motor 44. The tow 45 is formed and ispassed through the double-wall pipe 53 positioned along the axis of die52. Additional fibers are bonded to the outside of tow 45 to produce alarger tow 54 which may pass through a guard 55. Die 52 is also operatedwith forward takeoff. The tow 54 is then passed through the double-wallpipe 63 positioned along the axis of die 62. The operation of die 62 iswith reverse takeoff such that a smooth, bonded skin of fibers isproduced on the outer surface of the tow 64. The tow 64 may be wound ona reel (not shown). While the three dies 42, 52, and 62 have beenillustrated as two forward takeoff operations followed bya reversetakeoff, it is understood that when more than one die is used thepossible combinations increase with the number of dies used. I I

The present invention will be further described by the followingexamples which illustrate the present invention but are not to beconsidered as limitations to the invention.

Using a die having 240 die openings on a circle having a diameter of 4inches, polypropylene (30 melt flow rate) was melt blown in a devicesimilar to that illustrated in FIG. 1 using reverse takeoff. The rovingproduced was cut and tested as cigarette filters. The cut filterproducts were compared to a commercial filter made of cellulose acetate.The specific conditions used in making the roving and the specificroving products made are set forth in table I hereinafter:

TABLE I Example Commercial 1 2 3 4 5 fi er Conditions:

Polymer rate, gum/min l1. 4 17. 3 20. 0 Main air rate. o./min 1. 4191.067 1. 313 Main air/polymer rate. 56. 5 27. 99 29. 78 Aux. air rateNoJmin 0.933 0.793 1. 314 Temp. die. F. 668 680 688 Temp. air, F 664 690691 Takeofi, ft./min. 6. 53 10. 2 10.93 Twist rate, turns/inc 0. 534.441 305 Blade distance, inches 8 6 6 Blade shape, in MXfi 300.9% MXQVProduct:

Total denier 50, 100 50, 600 61,000 AP at 1,220 cc./min., mm. H1O 82.830.0 62.6 Percent tar removed 57. 5 32. 9 38. 7

' Stabilizer rod.

wall pipe 33 which supplies an auxiliary airstream from the gas plenum34 which is defined by the double-wall pipe 33. To begin the operation,a starter rod 35 is placed in the laydown zone 29 and is drawn away fromthe die to form a roving 36. The roving 36 is wrapped around a windupreel 37 which is rotated by a motor 38.

In FIGS. 3 and 4 the looping of a single fiber is illustrated with bothreverse takeoff and forward takeoff to illustrate the overall nature ofthe rovings produced by the melt blown roving technique. Of course, as aplurality of fibers are involved Comparing examples 1 and 2, the higherpressure drop through the filter or roving product and the percentage oftar removal illustrates the effect of the size of the fibers in theroving. The higher air rate and lower polymer rate of example 1 producessmaller fibers and consequently higher AP and higher percentage of tarremoval. In both examples I and 2, the overall denier of the filterproduct was approximately 50,000. Examples 3 and 4 illustrate the effectof fiber size in a filter product ata higher denier of approximately60,000 with the smaller fibers being produced in example 4 Example 5illustrates an optimum set ot conditions for obtaining a cigarettefilter material wherein a lower AP and higher tar removal is obtained ascompared to a commercial filter made of cellulose acetate Thecharacteristics of the roving may be changed by controlling the rotationof the stabilizer rod in the melt-blowing process. The rotation of thestabilizer rod produces in the roving a helical turn which can beexpressed in turns per inchv A more uniform and compacted roving isproduced when the roving has between 0.02 and 0.8 turns per inch. When aroving is produced without any twisting or with essentially no turns perinch, the roving structure is loose and often nonuniform. A rovingstructure which is firm and uniform is produced at approximately 0.5turns per inch. Wen the number of turns per inch approaches 1 the rovingstructure becomes very firm and may be too dense for use as a cigarettefilter material. Further, at a level of turns per inch approaching l ormore, there is a tendency to produce a very tight center core with aloose, untwisted outer surface. Thus, the uniformity of the twisting inthe roving may be completely lost at high rotation of the stabilizerrod. It is understood that while this is highly undesirable in producinga roving for a cigarette filter, this effect may be desired in a filtermaterial for some other use. In summary, the twist which is made a partof the roving is related to the crosssectional uniformity of the rovingproduct.

The outer surface characteristics of the roving are subject to a widevariation and may change the specific end use of the roving produced. Inproducing a roving for a cigarette filter, the proper outer surfacecharacteristics are a pliable, bonded web surface to produce a small,firm filter. A roving having the pliable, bonded web surface may be cutto a proper filter length without changing the roving characteristics. Aroving having an outer surface which is soft and unbonded appears fuzzyand lacks the necessary firmness for a cigarette filter. Further, uponcutting a soft or unbonded roving, the ends of such 158 material becomefrayed. If the outer surface of the roving has fused fiber agglomeratesproducing a harsh skin, the resulting filter product is rough,nonuniform and too firm, and cracking results when the filter materialis cut. The characteristics of the outer surface on the roving aregenerally controlled by the distance of the laydown zone from the circleof die openings or in other words by the focusing of the extruded fibersby control of the air to a point of contact of the fibers on the roving.If the extruded fibers are collected in a laydown zone very close to thedie openings, the outer surface of the roving may have a fused'fiberskin. On the other hand, if the fibers are collected in a laydown zoneat a great distance from the die openings, the outer surface of theroving may be fuzzy and uneven because the fibers are unbonded. In theexamples set forth in table I, a smooth, unifonn outer surface wasproduced using the reverse takeoff.

To further characterize the roving produced according to the presentinvention, reference is made again to FIGS. 3 and 4 which pictoriallyrepresent the layddtwn of a single fiber with reverse and forwardtakeoff respectively. This pictorial representation of a single fiberillustrates a general characterization of the roving even though it isunderstood that this representation is oversimplified. With themultiplicity of fibers extruded out of the die in the melt blownprocess, there is considerable entanglement of the fibers andself-bonding occurring throughout the roving. A generalizedcharacteristic of the roving of the present invention, however, isillustrated in FIGS. 3 and 4 which is a looping of the fibers in theform of repeating loops which may be less than an inch to 20 inches longand looping from a point on the axis or from a small radius therefrom toa larger radius or to the outer surface of the roving. As is illustratedin FIGS. 3 and 4, the point of contact of the continuous fiber as thefiber becomes a part of the overall roving structure is at the largerradius or outer surface with reverse takeoff (FIG. 3) while the point ofcontact is at the axis or smaller radius with forward takeoff (FIG. 4).In each instance, however, the roving produced with reverse or forwardtakeoff when pulled apart by tension along its axis breaks generallyalong the lines of the looped fibers to produce a conical break. I

A further characteristic of the roving of the present invention is thatthe fibers making up the roving structure are of a much smaller diameterthan the fibers used heretofore in making roving products. The averagediameter of the fibers in the roving product produced according to thepresent invention may be as small as 2 to microns.

In the melt-blown roving technique other thermoplastic resins may beused besides polypropylene such as the various nylons (6, 66 and 610),polystyrene, polyethylene terephthalate, polymethylmethacrylate, otherpolyolefins such as polyethylene, ethylene propylene copolymers. Byusing a partitioned die or when more than one die is used more than onekind of thermoplastic resin may be used in the melt-blown roving processof the present invention. Thus, the roving characteristics are subjectto modification by using a mixture of thermoplastic resins or alaminated tow may be formed of various resins. Further, additives orbinders may be incorporated into the roving by injection into one ormore of the airstreams. In this manner, composite structures can beformed with special, desirable properties such as charcoal impregnatedfilter rods.

The nature and objects of the present invention having been completelydescribed and illustrated and the best mode thereof set forth, what wewish to claim as new and useful and secure by Letters Patent is:

l. A roving of entangled and bonded thermoplastic fibers which comprisea plurality of essentially continuous thermoplastic fibers having anaverage diameter between 2 and 40 microns and said fibers being loopedback and forth essentially individually and independently from adjacentfibers from points near the center of said roving to points nearer theouter surface ofsaid roving in a fairly uniform manner.

2. A roving according to claim 1 wherein the roving is cylindrical.

3. A roving according to claim 1 wherein said fibers are selfbonded.

4. A roving according to claim 1 wherein said fibers are polypropylene.

5. A roving according to claim 4 having a bulk density within the rangeof between 0.05 and 0.20 g./cc.

6. A roving according to claim 4 having a bulk density within the rangeof between 0.09 and 0.16 g./cc.

7. A roving according to claim 4 having a smooth,.firm

outer surface.

8. A roving according to claim 5 wherein the roving is cylindrical.

9. A roving according to claim 8 having a circumference within the rangeof 24 to 28 mm.

10. A cigarette filter made from the roving of claim 4. 11. A cigarettefilter made from the roving of claim 9.

2. A roving according to claim 1 wherein the roving is cylindrical.
 3. Aroving according to claim 1 wherein said fibers are self-bonded.
 4. Aroving according to claim 1 wherein said fibers are polypropylene.
 5. Aroving according to claim 4 having a bulk density within the range ofbetween 0.05 and 0.20 g./cc.
 6. A roving according to claim 4 having abulk density within the range of between 0.09 and 0.16 g./cc.
 7. Aroving according to claim 4 having a smooth, firm outer surface.
 8. Aroving according to claim 5 wherein the roving is cylindrical.
 9. Aroving according to claim 8 having a circumference within the range of24 to 28 mm.
 10. A cigarette filter made from the roving of claim
 4. 11.A cigarette filter made from the roving of claim 9.